Optical fiber

ABSTRACT

The present invention relates to an optical fiber having a large positive dispersion in a wavelength band of 1.55 μm in order to compensate for a negative dispersion inherent in an NZ-DSF in the wavelength band of 1.55 μm. This optical fiber comprises a depressed cladding structure constituted by a core region; an inner cladding, disposed at the outer periphery of the core region, having a lower refractive index; and an outer cladding having a higher refractive index. In this optical fiber, the relative refractive index difference of the core region with respect to the outer cladding is at least 0.30% but not greater than 0.50%, and the relative refractive index difference of the inner cladding with respect to the outer cladding is at least −0.50% but not greater than −0.02%. Also, the optical fiber has a dispersion greater than 18 ps/nm/km at a wavelength of 1.55 μm, and an effective cross-sectional area A eff  of at least 70 μm 2  at the wavelength of 1.55 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relate to an optical fiber applicable to a moduleinstalled in a part of an optical transmission line or on the opticaltransmission line in an optical transmission system which carries outWDM communications mainly in a 1.55-μm wavelength band.

2. Related Background Art

WDM (Wavelength Division Multiplexing) communication systems enablelarge-capacity, high-speed optical communications by transmitting aplurality of signal light components in the 1.55-μm wavelength band(1.53 μm to 1.57 μm). Since optical transmission systems carrying outsuch WDM communications preferably have a low dispersion in the 1.55-μmwavelength band so as to be able to transmit signal light in a widewavelength band, a dispersion-shifted optical fiber whosezero-dispersion wavelength is shifted to the 1.55-μm wavelength band(DSF: Dispersion Shifted Fiber) has been utilized in their opticaltransmission lines.

If the dispersion in the 1.55-μm wavelength band is substantially zero,however, then four-wave mixing, which is a kind of nonlinear opticalphenomena, may occur, whereby the signal light at the time of receptionis likely to deteriorate (see, for example, H. Taga, et al., OFC'98,PD13). Therefore, a dispersion-shifted optical fiber whosezero-dispersion wavelength is further shifted to the longer wavelengthside so that the dispersion at a wavelength of 1.55 μm is set to about−2 ps/nm/km (no zero-dispersion wavelength exists in the signalwavelength band) (NZ-DSF: Non-zero Dispersion Shifted Fiber) hasconventionally been employed in optical transmission lines, so as tosuppress the four-wave mixing. Since the above-mentioned NZ-DSF has anegative dispersion in the 1.55-μm wavelength band, there are caseswhere a dispersion-compensating optical fiber having a positivedispersion in the 1.55-μm wavelength band is employed in an opticaltransmission line together with the NZ-DSF (see, for example, M. Suzuki,et at., OFC'98, PD17).

As the above-mentioned dispersion-compensating optical fiber, opticalfibers defined by G652 and G654 standards of ITU-T, for example, havebeen known. The optical fiber of G652 standard is a regular opticalfiber constituted by a core region made of Ge-doped silica and acladding region made of pure silica. This optical fiber of G652 standardhas a zero-dispersion wavelength in a 1.3-μm wavelength band and adispersion of about 17 ps/nm/km in the 1.55-μm wavelength band. On theother hand, the optical fiber of G654 standard has a dispersion of 20ps/nm/km or less in the 1.55-μm wavelength band. Further, an opticalfiber, constituted by a core region made of pure silica and a claddingregion made of F-doped silica, having a dispersion of about 18 ps/nm/kmin the 1.55-μm wavelength band is also used as a dispersion-compensatingoptical fiber.

Since a conventional optical transmission line thus constituted by theNZ-DSF and the dispersion-compensating optical fiber has a positivedispersion slope as a whole, though the dispersion becomes zero in onewavelength in the 1.55-μm wavelength band, it does not become zero inthe other wavelength regions. Therefore, in order to compensate for theresidual dispersion in the other wavelength regions, the signal light inthe other wavelength regions is demultiplexed in a base station or thelike, so that the dispersion of each signal light component iscompensated for by use of a dispersion-compensating optical fiber ofG652 or G654 standard. Here, the dispersion slope is given by thegradient of the curve indicating the dependence of the dispersion uponwavelength.

SUMMARY OF THE INVENTION

As a result of studies concerning the above-mentioned prior art, theinventors have found the following problems. Namely, since the upperlimit of dispersion in the 1.55-μm wavelength band exceeds 20 ps/nm/kmin the above-mentioned dispersion-compensating optical fiber of G654standard, it is needed to be elongated so as to compensate for thenegative dispersion inherent in the NZ-DSF in the 1.55-μm wavelengthband. Also, in optical fibers having a simple step-like refractive indexprofile composed of a core region and a cladding region, the upper limitof dispersion is determined according to the upper limit of cutoffwavelength, whereby it is difficult to enhance the dispersion in the1.55-μm wavelength band.

In order to overcome the problems such as those mentioned above, it isan object of the present invention to provide an optical fiber which hasa large positive dispersion in the 1.55-μm wavelength band, andcompensates for the negative distribution inherent in the NZ-DSF in the1.55-μm wavelength band.

The optical fiber according to the present invention comprises a coreregion extending along a predetermined axis, and a cladding regiondisposed at the outer periphery of the core region. The cladding regionhas a depressed cladding structure comprising an inner cladding which isa region disposed at the outer periphery of the core region, and anouter cladding which is a region disposed at the outer periphery of theinner cladding and has a refractive index lower than that of the coreregion but higher than that of the inner cladding. Also, in this opticalfiber, the relative refractive index difference of the core region withrespect to the outer cladding is at least 0.30% but not greater than0.50%, and the relative refractive index difference of the innercladding with respect to the outer cladding is at least −0.50% but notgreater than −0.02%. At a wavelength of 1.55 μm, the optical fiber has adispersion greater than 18 ps/nm/km and an effective cross-sectionalarea A_(eff) of at least 70 μm².

As indicated in Japanese Patent Application Laid-Open No.8-248251 (EP 0724171 A2), the effective cross-sectional area A_(eff) is given by thefollowing expression (1): $\begin{matrix}{A_{eff} = {2{{\pi \left( {\int_{0}^{\infty}{E^{2}r\quad {r}}} \right)}^{2}/\left( {\int_{0}^{\infty}{E^{4}r\quad {r}}} \right)}}} & (1)\end{matrix}$

where E is the electric field accompanying the propagated light, and ris the radial distance from the core center.

Since this optical fiber has a large dispersion in the 1.55-μmwavelength band as such, a short length is sufficient when compensatingfor the negative dispersion inherent in the NZ-DSF in the 1.55-μmwavelength band. As a consequence, it is favorable in that, when theoptical fiber is wound at a predetermined diameter so as to form amodule, the resulting module can be made smaller. Also, since theeffective cross-sectional area at the wavelength of 1.55 μm is large,nonlinear optical phenomena can effectively be restrained fromoccurring. In addition to the characteristics mentioned above, theoptical fiber according to the present invention preferably has adispersion of 20 ps/nm/km or greater at the wavelength of 1.55 μm. Sincethis optical fiber has a greater dispersion in the 1.55-μm wavelengthband, it can be made shorter when compensating for the negativedispersion inherent in the NZ-DSF in the 1.55-μm wavelength band,whereby it becomes easier to reduce the dimensions of adispersion-compensating module to which the optical fiber is applied. Inparticular, for realizing various characteristics at the wavelength of1.55 μm, each of the optical fibers having the configurations mentionedabove preferably satisfies the relationships of:

2.0≦2b/2a≦6.0

8.3≦2a≦13.0

where 2 a (unit: μm) is the outside diameter of the core region, and 2 b(unit: μm) is the outside diameter of the inner cladding.

The optical fiber according to the present invention may have aconfiguration comprising a core region which extends along apredetermined axis and has an outside diameter of at least 9.5 μm butnot greater than 13.0 μm, and a cladding region having a refractiveindex lower than that of the core region. In such a configuration, therelative refractive index difference of the core region with respect tothe cladding region is at least 0.3% but not greater than 0.5%. Also,the dispersion at the wavelength of 1.55 μm is at least 20 ps/nm/km, andthe effective cross-sectional area A_(eff) at the wavelength of 1.55 μmis at least 70 μm². Since this optical fiber also has a large dispersionin the 1.55-μm wavelength band, a short length is sufficient whencompensating for the negative dispersion inherent in the NZ-DSF in the1.55-μm wavelength band. Also, since the effective cross-sectional areaat the wavelength of 1.55 μm is large, nonlinear optical phenomena areeffectively restrained from occurring.

Preferably, each of the optical fibers having various configurationsmentioned above has a transmission loss of 0.215 dB/km or less at thewavelength of 1.55 μm when wound like a coil at a diameter of 60 mm, anda polarization mode dispersion of 0.25 ps·km^(−½) or less at thewavelength of 1.55 μm. In this case, sufficient characteristics can beobtained in the optical fiber according to the present invention even ina configuration in which it is wound like a coil so as to form a module.

As a further preferred optical characteristic, the optical fiberaccording to the present invention has an effective cross-sectional areaA_(eff) of 90 μm² or greater. Also, this optical fiber has a cutoffwavelength of 1.4 μm or greater at a fiber length of 2 m. Further, thisoptical fiber has a transmission loss of 0.180 dB/km or less at thewavelength of 1.55 μm.

The inventors have experimentally confirmed that providing a carboncoating on the surface of the optical fiber according to the presentinvention is effective in preventing the optical fiber from breaking.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing a cross-sectional structure of a firstembodiment of the optical fiber according to the present invention,whereas FIG. 1B is a chart showing a refractive index profile of theoptical fiber shown in FIG. 1A;

FIG. 2 is a graph showing relationships between the core diameter(outside diameter of the core region) and the dispersion at a wavelengthof 1550 nm in the optical fiber according to the first embodiment in thecase where the ratio (2 b/2 a) of the outside diameter 2 b of the innercladding to the outside diameter 2 a of the core region is fixed at 4.0,whereas the relative refractive index difference Δ⁻ of the innercladding with respect to the outer cladding is fixed at −0.03%;

FIG. 3 is a graph showing relationships between the core diameter(outside diameter of the core region) and the dispersion at thewavelength of 1550 nm in the optical fiber according to the firstembodiment in the case where the ratio (2 b/2 a) of the outside diameter2 b of the inner cladding to the outside diameter 2 a of the core regionis fixed at 4.0, whereas the relative refractive index difference Δ⁻ ofthe inner cladding with respect to the outer cladding is fixed at−0.09%;

FIG. 4 is a graph showing relationships between the core diameter(outside diameter of the core region) and the dispersion at thewavelength of 1550 nm in the optical fiber according to the firstembodiment in the case where the ratio (2 b/2 a) of the outside diameter2 b of the inner cladding to the outside diameter 2 a of the core regionis fixed at 4.0, whereas the relative refractive index difference Δ⁻ ofthe inner cladding with respect to the outer cladding is fixed at−0.20%;

FIG. 5 is a graph showing relationships between the core diameter(outside diameter of the core region) and the dispersion at thewavelength of 1550 nm in the optical fiber according to the firstembodiment in the case where the ratio (2 b/2 a) of the outside diameter2 b of the inner cladding to the outside diameter 2 a of the core regionis fixed at 4.0, whereas the relative refractive index difference Δ⁻ ofthe inner cladding with respect to the outer cladding is fixed at−0.45%;

FIG. 6A is a chart showing the refractive index profile of an appliedexample of the optical fiber according to the first embodiment, whereasFIG. 6B is a chart showing the refractive index profile of anotherapplied example of the optical fiber according to the first embodiment;

FIG. 7 is a graph showing results of experiments for explaining thebreaking prevention effect obtained by carbon coating;

FIG. 8A is a view showing a cross-sectional structure of a secondembodiment of the optical fiber according to the present invention,whereas FIG. 8B is a chart showing a refractive index profile of theoptical fiber shown in FIG. 8A; and

FIG. 9 is a graph showing the relationship between the core diameter(outside diameter of the core region) 2 a and the dispersion at thewavelength of 1550 nm in the optical fiber according to the secondembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the optical fiber according to thepresent invention will be explained with reference to FIGS. 1A, 1B, 2 to5, 6A, 6B, 7, 8A, 8B, and 9. Among the drawings, constituents identicalto each other will be referred to with numerals or letters identical toeach other, without repeating their overlapping descriptions.

(First Embodiment)

FIG. 1A is a view showing a cross-sectional structure of the opticalfiber according to the first embodiment, whereas FIG. 1B is a refractiveindex profile of the optical fiber shown in FIG. 1A. The optical fiber100 according to the first embodiment comprises a core region 110extending along a predetermined axis and having a refractive index n₁and an outside diameter 2 a (μm), and a cladding region disposed at theouter periphery of the core region 110. For realizing a depressedcladding structure, the cladding region further has an inner cladding120, which is a region disposed at the outer periphery of the coreregion 110 and has a refractive index n₂ (<n₁) and an outside diameter 2b, and an outer cladding 130, which is a region disposed at the outerperiphery of the inner cladding 120 and has a refractive index n₃ (<n₁,>n₂). Therefore, the respective refractive indices in the regions 110,120, and 130 have a relationship of n₁>n₃>n₂ in terms of magnitude. Theouter periphery of the optical fiber 100 according to the firstembodiment is provided with a carbon coating 140 for effectivelypreventing the fiber from breaking when it is wound like a coil so as toform a module.

The abscissa of the refractive index profile 150 shown in FIG. 1Bcorresponds to individual parts along the line L in FIG. 1A on a crosssection perpendicular to the center axis of the core region 110.Therefore, in the refractive index profile 150 of FIG. 1B, regions 151,152, and 153 indicate the respective refractive indices in individualparts on the line L in the core region 110, inner cladding 120, andouter cladding 130.

The optical fiber having such a refractive index profile 150 is asingle-mode optical fiber based on silica, which can be realized, forexample, by adding Ge and F elements to the core region 110 and theinner cladding 120, respectively. In FIGS. 1A and 1B, 2 a indicates theoutside diameter of the core region 110, whereas 2 b indicates theoutside diameter of the inner cladding 120. Δ⁺ and Δ⁻ indicate therespective relative refractive index differences of the core region 110and inner cladding region 120 with respect to the outer cladding 130.Here, the relative refractive index difference Δ⁺ of the core region 110with respect to the outer cladding 130 and the relative refractive indexdifference Δ⁻ of the inner cladding 120 with respect to the outercladding 130 are defined respectively as follows:

Δ⁺=(n ₁ −n ₃)/n ₃

Δ⁻=(n ₂ −n ₃)/n ₃

where n₁ is the refractive index of the core region 110, n₂ is therefractive index of the inner cladding 120, and n₃ is the refractiveindex of the outer cladding 130. In this specification, the relativerefractive index difference Δ is represented by percentage, and therespective refractive indices of individual regions in each definingexpression may be arranged in any order. Consequently, the case where Δis a negative value indicates that the refractive index of itscorresponding region is lower than the that of the outer cladding 130.

In the optical fiber 100 according to the first embodiment, the relativerefractive index difference Δ⁺ of the core region 110 with respect tothe outer cladding 130 is at least 0.30% but not greater than 0.50%,whereas the relative refractive index difference Δ⁻ of the innercladding 120 with respect to the outer cladding 130 is at least −0.50%but not greater than −0.02%. Also, the dispersion at the wavelength of1.55 μm is greater than 18 ps/nm/km, and the effective cross-sectionalarea A_(eff) at the wavelength of 1.55 μm is at least 70 μm².

FIGS. 2 to 5 are graphs each showing relationships between the outsidediameter 2 a of the core region 110 according to the first embodimentand its dispersion at the wavelength of 1.55 μm. Here, in the graph ofFIG. 2, the ratio (2 b/2 a) of the outside diameter 2 b of the innercladding 120 to the outside diameter 2 a of the core region 110 and therelative refractive index difference Δ⁻ of the inner cladding 120 withrespect to the outer cladding 130 are fixed at 4.0 and −0.03%,respectively. Also, in the graph of FIG. 3, the ratio (2 b/2 a) of theoutside diameter 2 b of the inner cladding 120 to the outside diameter 2a of the core region 110 and the relative refractive index difference Δ⁻of the inner cladding 120 with respect to the outer cladding 130 arefixed at 4.0 and −0.09%, respectively. In the graph of FIG. 4, the ratio(2 b/2 a) of the outside diameter 2 b of the inner cladding 120 to theoutside diameter 2 a of the core region 110 and the relative refractiveindex difference Δ⁻ of the inner cladding 120 with respect to the outercladding 130 are fixed at 4.0 and −0.20%, respectively. Further, in thegraph of FIG. 5, the ratio (2 b/2 a) of the outside diameter 2 b of theinner cladding 120 to the outside diameter 2 a of the core region 110and the relative refractive index difference Δ⁻ of the inner cladding120 with respect to the outer cladding 130 are fixed at 4.0 and −0.45%,respectively.

In each of FIGS. 2 to 5, G100, G200, and G300 are curves indicating therelationships between the core diameter 2 a and the dispersion value atthe wavelength of 1.55 μm in the cases where the relative refractiveindex difference Δ⁺ of the core region 110 with respect to the outercladding 130 is 0.50%, 0.40%, and 0.30%, respectively. C1 is a curveshowing the relationship between the core diameter 2 a and thedispersion value at the wavelength of 1.55 μm, in which the increase inloss (at the wavelength of 1.55 μm) in the optical fiber having a totallength of 20 km caused by being wound at a diameter of 60 mm becomes0.01 dB/km. Further, each of FIGS. 2 to 5 shows curves indicating therelationships between the core diameter 2 a and the dispersion value atthe wavelength of 1.55 μm in the cases where the cutoff wavelength λcbecomes 1.5 μm and 1.6 μm, respectively; and the relationships betweenthe core diameter 2 a and the dispersion value at the wavelength of 1.55μm in the cases where the effective cross-sectional area A_(eff) becomes70 μm², 80 μm², and 90 μm², respectively. A cutoff wavelength λc up toabout 1.60 μm is permissible in the case of an optical fiber having alength of several hundreds of meters, and that up to about 1.70 μm maybe permissible in the case of a longer optical fiber. In each of FIGS. 2to 5, an area where the cutoff wavelength λc is 1.6 μm or shorter, theeffective cross-sectional area A_(eff) is at least 70 μm², and theincrease in loss (at the wavelength of 1.55 μm) in the optical fiberhaving a total length of 20 km caused by being wound at a diameter of 60mm becomes 0.01 dB/km or less is indicated as a preferable range(hatched area in each graph).

Judging from FIG. 2, in the optical fiber in which the relativerefractive index difference Δ⁻ of the inner cladding 120 with respect tothe outer cladding 130 is −0.03%, when the outside diameter 2 a of thecore region 110 is about 8.3 μm or greater, then the dispersion at thewavelength of 1.55 μm can become about 18 ps/nm/km or greater. When theoutside diameter 2 a of the core region 110 is about 9.2 μm or greater,then the dispersion at the wavelength of 1.55 μm can become about 20ps/nm/km or greater. Also, when the outside diameter 2 a of the coreregion 110 is about 12.5 μm, then the dispersion at the wavelength of1.55 μm can be increased up to about 21.3 ps/nm/km.

Judging from FIG. 3, in the optical fiber in which the relativerefractive index difference Δ⁻ of the inner cladding 120 with respect tothe outer cladding 130 is −0.09%, when the outside diameter 2 a of thecore region 110 is about 8.3 μm or greater, then the dispersion at thewavelength of 1.55 μm can become about 18 ps/nm/km or greater. When theoutside diameter 2 a of the core region 110 is about 9.1 μm or greater,then the dispersion at the wavelength of 1.55 μm can become about 20ps/nm/km or greater. Also, when the outside diameter 2 a of the coreregion 110 is about 12.5 μm, then the dispersion at the wavelength of1.55 μm can be increased up to about 21.7 ps/nm/km.

Also, judging from FIG. 4, in the optical fiber in which the relativerefractive index difference Δ⁻ of the inner cladding 120 with respect tothe outer cladding 130 is −0.20%, when the outside diameter 2 a of thecore region 110 is about 9.5 μm or greater, then the dispersion at thewavelength of 1.55 μm can become about 20.8 ps/nm/km or greater. Also,when the outside diameter 2 a of the core region 110 is about 12.8 μm,then the dispersion at the wavelength of 1.55 μm can be increased up toabout 22.3 ps/nm/km.

Further, judging from FIG. 5, in the optical fiber in which the relativerefractive index difference Δ⁻ of the inner cladding 120 with respect tothe outer cladding 130 is −0.45%, when the outside diameter 2 a of thecore region 110 is about 10.5 μm or greater, then the dispersion at thewavelength of 1.55 μm can become about 23.2 ps/nm/km or greater. Also,when the outside diameter 2 a of the core region 110 is about 13.0 μm,then the dispersion at the wavelength of 1.55 μm can be increased up toabout 23.5 ps/nm/km.

As can be seen from FIGS. 2 to 5 in the foregoing, when the relativerefractive index difference Δ⁻ of the inner cladding 120 with respect tothe outer cladding 130 is reduced (its absolute value is increased),then the dispersion can be enhanced while keeping the cutoff wavelengthλc at the same value.

A plurality of applied examples of the optical fiber according to thefirst embodiment will now be explained.

To begin with, the optical fiber 100 according to a first appliedexample has the cross-sectional structure shown in FIG. 1A and therefractive index profile shown in FIG. 1B, whereas the outside diameter2 a of the core region 110, the outside diameter 2 b of the innercladding 120, the relative refractive index difference Δ⁺ of the coreregion 110 with respect to the outer cladding 130, and the relativerefractive index difference Δ⁻ of the inner cladding 120 with respect tothe outer cladding 130 are set as follows:

2 a (μm): 9.0

2 b (μm): 36.0

Δ⁺(%): 0.35

Δ⁻(%): 0.03

Thus designed optical fiber according to the first applied example has,as various characteristics at the wavelength of 1.55 μm, the followingoptical characteristics:

dispersion (ps/nm/km): 18.7

effective cross-sectional area A_(eff) (μm²): 80.5

dispersion slope (ps/nm²/km): 0.058

transmission loss (dB/km)

when bent at a diameter of 60 mm:: 0.208

polarization mode dispersion PMD (ps·km^(½)): 0.14

Here, the cutoff wavelength of the optical fiber according to the firstapplied example at a length of 2 m is 1.25 μm. Also, the above-mentionedtransmission loss is the sum of the original transmission loss of theoptical fiber and the increase in loss caused by being bent at thediameter of 60 mm.

The optical fiber according to a second applied example also has thecross-sectional structure shown in FIG. 1A, and its refractive indexprofile has a form similar to that shown in FIG. 1B. Also, this opticalfiber of the second applied example is designed with the followingfeatures:

2 a (μm): 10.5

2 b (μm): 42.0

Δ⁺(%): 0.35

Δ⁻(%): −0.03

Thus designed optical fiber according to the second applied example has,as various characteristics at the wavelength of 1.55 μm, the followingoptical characteristics:

dispersion (ps/nm/km): 20.4

effective cross-sectional area A_(eff) (μm²): 93.2

dispersion slope (ps/nm²/km): 0.060

transmission loss (dB/km)

when bent at a diameter of 60 mm:: 0.204

polarization mode dispersion PMD (ps·km^(½)): 0.12

Here, the cutoff wavelength of the optical fiber according to the secondapplied example at a length of 2 m is 1.45 μm. Also, the above-mentionedtransmission loss is the sum of the original transmission loss of theoptical fiber and the increase in loss caused by being bent at thediameter of 60 mm.

The optical fiber according to a third applied example is designed withthe following features:

2 a (μm): 10.5

2 b (μm): 46.0

Δ⁺(%): 0.35

Δ⁻(%): −0.03

Thus designed optical fiber according to the third applied example has,as various characteristics at the wavelength of 1.55 μm, the followingoptical characteristics:

dispersion (ps/nm/km): 21.0

effective cross-sectional area A_(eff) (μm²): 103.0

dispersion slope (ps/nm²/km): 0.061

transmission loss (dB/km)

when bent at a diameter of 60 mm:: 0.202

polarization mode dispersion PMD (ps·km^(½)): 0.12

Here, the cutoff wavelength of the optical fiber according to the thirdapplied example at a length of 2 m is 1.59 μm. Also, the above-mentionedtransmission loss is the sum of the original transmission loss of theoptical fiber and the increase in loss caused by being bent at thediameter of 60 mm.

Further, the optical fiber according to a fourth applied example isdesigned with the following features:

2 a (μm): 10.0

2 b (μm): 40.0

Δ⁺(%): 0.31

Δ⁻(%): −0.03

Thus designed optical fiber according to the fourth applied example has,as various characteristics at the wavelength of 1.55 μm, the followingoptical characteristics:

dispersion (ps/nm/km): 19.6

effective cross-sectional area A_(eff) (μm²): 98.0

dispersion slope (ps/nm²/km): 0.060

transmission loss (dB/km)

when bent at a diameter of 60 mm:: 0.204

polarization mode dispersion PMD (ps·km^(½)): 0.12

Here, the cutoff wavelength of the optical fiber according to the fourthapplied example at a length of 2 m is 1.31 μm. Also, the above-mentionedtransmission loss is the sum of the original transmission loss of theoptical fiber and the increase in loss caused by being bent at thediameter of 60 mm.

The optical fiber according to a fifth applied example has thecross-sectional structure shown in FIG. 1A and a refractive indexprofile 160 shown in FIG. 6A. As can also be seen from the form of therefractive index profile 160, in the fifth applied example, the coreregion 110 has such a form that the center part thereof is depressedfrom its surroundings and the skirt portions of the core region 110 havean inclined form (form in which the skirt portions extend toward theinner cladding 120). The abscissa of this refractive index profile 160corresponds to individual parts along the line L in FIG. 1A on a crosssection perpendicular to the center axis of the core region 110.Therefore, in the refractive index profile 160, regions 161, 162, and163 indicate the respective refractive indices in individual parts onthe line L in the core region 110 (having the outside diameter 2 a),inner cladding 120 (having the outside diameter 2 b), and outer cladding130. Here, in the fifth applied example, the relative refractive indexdifference Δ⁺ of the core region 110 with respect to the outer cladding130 is given by the refractive index n₃ of the outer cladding 130 andthe average refractive index n₁ of the core region 110, whereas therelative refractive index difference Δ⁻ of the inner cladding 120 withrespect to the outer cladding 130 is given by the refractive index n₃ ofthe outer cladding 130 and the minimum refractive index n₂ of the innercladding 120.

Such an optical fiber according to the fifth applied example is designedaccording to the following features:

2 a (μm): 10.0

2 b (μm): 45.4

Δ⁺(%): 0.34

Δ⁻(%): −0.03

Thus designed optical fiber according to the fifth applied example has,as various characteristics at the wavelength of 1.55 μm, the followingoptical characteristics:

dispersion (ps/nm/km): 19.5

effective cross-sectional area A_(eff) (μm²): 105.0

dispersion slope (ps/nm²/km): 0.062

transmission loss (dB/km)

when bent at a diameter of 60 mm:: 0.198

polarization mode dispersion PMD (ps·km^(½)): 0.13

Here, the cutoff wavelength of the optical fiber according to the fifthapplied example at a length of 2 m is 1.62 μm. Also, the above-mentionedtransmission loss is the sum of the original transmission loss of theoptical fiber and the increase in loss caused by being bent at thediameter of 60 mm.

The optical fiber according to a sixth applied example has thecross-sectional structure shown in FIG. 1A and a refractive indexprofile 170 shown in FIG. 6B. This sixth applied example comprises acore region 110 made of pure silica, and an inner cladding 120 and anouter cladding 130 which are made of F-doped silica. The abscissa ofthis refractive index profile 170 corresponds to individual parts alongthe line L in FIG. 1A on a cross section perpendicular to the centeraxis of the core region 110. Therefore, in the refractive index profile170, regions 171, 172, and 173 indicate the respective refractiveindices in individual parts on the line L in the core region 110 (havingthe outside diameter 2 a), inner cladding 120 (having the outsidediameter 2 b), and outer cladding 130.

Such an optical fiber according to the sixth applied example is designedaccording to the following features:

2 a (μm): 11.6

2 b (μm): 46.4

Δ⁺(%): 0.31

Δ⁻(%): −0.05

Thus designed optical fiber according to the sixth applied example has,as various characteristics at the wavelength of 1.55 μm, the followingoptical characteristics:

dispersion (ps/nm/km): 20.5

effective cross-sectional area A_(eff) (μm²): 99

dispersion slope (ps/nm²/km): 0.060

transmission loss (dB/km)

when bent at a diameter of 60 mm:: 0.172

polarization mode dispersion PMD (ps·km^(½)): 0.08

Here, the cutoff wavelength of the optical fiber according to the sixthapplied example at a length of 2 m is 1.50 μm. Also, the above-mentionedtransmission loss is the sum of the original transmission loss of theoptical fiber and the increase in loss caused by being bent at thediameter of 60 mm.

In addition, optical fibers were designed or prototyped under variousconditions, and their various characteristics were evaluated. As aresult, it has been found that sufficiently large dispersion andeffective cross-sectional area A_(eff) are obtained at the wavelength of1.55 μm. In particular, it has been found preferable to satisfy therelational expression of 2.0≦2 b/2 a≦6.0, where 2 a (unit: μm) is theoutside diameter of the core region, and 2 b (unit: μm) is the outsidediameter of the inner cladding region. Also, it has been confirmed thatthe transmission loss (the sum of the original transmission loss of theoptical fiber and the increase in loss caused by bending) at thewavelength of 1.55 μm when wound like a coil at a diameter of 60 mmbecomes 0.215 dB/km or less in the optical fiber according to the firstembodiment, and that the original transmission loss of the optical fiberat the wavelength of 1.55 μm becomes 0.180 dB/km or less in furtherpreferable applied examples thereof. Further, it has been found that thepolarization mode dispersion at the wavelength of 1.55 μm is 0.25ps·km^({fraction (−1/2)}) or less in the optical fiber according to thefirst embodiment.

Meanwhile, the inventors have experimentally confirmed that providing acarbon coating on the surface of the above-mentioned optical fiber iseffective in preventing the optical fiber from breaking.

FIG. 7 is a graph showing results of experiments for explaining thebreaking prevention effect obtained by carbon coating, in which curveG400 indicates the relationship between the pulling rate (mm/min) andthe tensile strength (GPa) when an optical fiber provided with a carboncoating is broken, and graph G500 indicates the relationship between thepulling rate (mm/min) and the tensile strength (GPa) when an opticalfiber provided with no carbon coating is broken. Also, while the fatigueindex N of the optical fiber provided with the carbon coating exceeded150, that of the optical fiber provided with no carbon coating was about25. Here, the breaking strength (Gpa) at the time when the optical fiberis broken has been known to be proportional to the pulling rate(mm/min), at which the optical fiber is pulled, to the [1/(N+1)]-thpower as follows:

(breaking strength)=α×(pulling rate)^(1/(N+1))

where N in the expression is particularly referred to as fatigue index.

As can also be seen from FIG. 7, the difference in breaking strengthcaused by whether there is a carbon coating or not becomes smaller asthe pulling rate increases (i.e., apparently, when pulled faster, flawsare less likely to grow and the fiber is less likely to break even ifthe same force is applied thereto). However, since actually laid opticalfibers are caused to break as being pulled at a very low rate, theoptical fiber provided with a carbon coating having a high breakingstrength at a low pulling rate is further preferable.

As explained in the foregoing, since the optical fiber according to thefirst embodiment has a large positive dispersion in the wavelength bandof 1.55 μm, it needs only a short length for compensating for thenegative dispersion inherent in the NZ-DSF in the wavelength band of1.55 μm, thus making it possible to reduce the dimensions of adispersion-compensating module to which this optical fiber is applied.Also, since this optical fiber has a large effective cross-sectionalarea A_(eff) at the wavelength of 1.55 μm, nonlinear optical phenomenaare effectively restrained from occurring. Further, since this opticalfiber has a low transmission loss at the wavelength of 1.55 μm whenwound like a coil at a diameter of 60 mm, and its polarization modedispersion at the wavelength of 1.55 μm is small, it is suitable forforming a module.

(Second Embodiment)

The second embodiment of the optical fiber according to the presentinvention will now be explained. FIG. 8A is a view showing across-sectional structure of the optical fiber according to the secondembodiment, whereas FIG. 8B is a refractive index profile of the opticalfiber shown in FIG. 8A. The optical fiber 200 according to the secondembodiment comprises a core region 210 which extends along apredetermined axis and has a refractive index n₁, and a cladding region220 which is a region disposed at the outer periphery of the core region210 and has a refractive index n₂ (<n₁). As a consequence, therelationship of the respective refractive indices of the regions 210,220 in terms of magnitude is n₁>n₂. The outer periphery of the opticalfiber 200 according to the second embodiment is provided with a carboncoating 230 in order to effectively prevent the fiber from breaking whenformed into a module by being wound like a coil.

The abscissa of the refractive index profile 250 shown in FIG. 8Bcorresponds to individual parts along the line L in FIG. 8A on a crosssection perpendicular to the center axis of the core region 210.Therefore, in the refractive index profile 250 of FIG. 8B, regions 251and 252 indicate the respective refractive indices in individual partson the line L in the core region 210 and cladding region 220.

The optical fiber having such a refractive index profile 250 is asingle-mode optical fiber based on silica, which can be realized, forexample, by adding Ge element to the core region 210. It can also berealized by making the core region 210 with pure silica and adding Felement to the cladding region 220. In FIGS. 8A and 8B, 2 a indicatesthe outside diameter of the core region 210, whereas Δ⁺ indicates therelative refractive index difference of the core region 210 with respectto the cladding region 220.

Also, in the optical fiber 200 according to the second embodiment, therelative refractive index difference Δ⁺ (=(n₁−n₂)/n₂) of the core region210 with respect to the cladding region 220 is at least 0.3% but notgreater than 0.5%, the dispersion at the wavelength of 1.55 μm is atleast 20 ps/nm/km, the effective cross-sectional area at the wavelengthof 1.55 μm is at least 70 μm², and the outside diameter of the coreregion 210 is at least 9.5 μm but not greater than 12.0 μm.

FIG. 9 is a graph showing relationships between the outside diameter 2 aof the core region 210 according to the second embodiment and itsdispersion at the wavelength of 1.55 μm. In this graph, G100, G200, andG300 are curves indicating the relationships between the core diameter 2a and the dispersion value at the wavelength of 1.55 μm in the caseswhere the relative refractive index difference Δ⁺ of the core region 210with respect to the cladding region 220 is 0.30%, 0.40%, and 0.50%,respectively. C1 is a curve showing the relationship between the corediameter 2 a and the dispersion value at the wavelength of 1.55 μm, inwhich the increase in loss (at the wavelength of 1.55 μm) in the opticalfiber having a total length of 20 km caused by being wound at a diameterof 60 mm becomes 0.01 dB/km. Further, FIG. 9 shows curves indicating therelationships between the core diameter 2 a and the dispersion value atthe wavelength of 1.55 μm in the cases where the cutoff wavelength λcbecomes 1.5 μm and 1.6 μm, respectively; and the relationships betweenthe core diameter 2 a and the dispersion value at the wavelength of 1.55μm in the cases where the effective cross-sectional area A_(eff) becomes70 μm², 80 μm², and 90 μm², respectively. A cutoff wavelength λc up toabout 1.60 μm is permissible in the case of an optical fiber having alength of several hundreds of meters, and that up to about 1.70 μm maybe permissible in the case of a longer optical fiber. In FIG. 9, an areawhere the cutoff wavelength λc is 1.6 μm or shorter, the effectivecross-sectional area A_(eff) is at least 70 μm², and the increase inloss (at the wavelength of 1.55 μm) in the optical fiber having a totallength of 20 km caused by being wound at a diameter of 60 mm becomes0.01 dB/km or less is indicated as a preferable range (hatched area inthe graph).

Judging from FIG. 9, when the outside diameter 2 a of the core region210 is about 9.5 μm or greater, then the dispersion at the wavelength of1.55 μm can become about 20 ps/nm/km or greater. When the outsidediameter 2 a of the core region 210 is about 12.0 μm, then thedispersion at the wavelength of 1.55 μm can be increased up to about20.7 ps/nm/km.

In the optical fiber 200 according to the second embodiment, the outsidediameter 2 a of the core region 210 is 11.0 μm, and the relativerefractive index difference Δ⁺ of the core region 210 with respect tothe cladding region 220 is 0.35%. At this time, the cutoff wavelength λcwas 1.54 μm, the dispersion at the wavelength of 1.55 μm was 20.3ps/nm/km, the effective cross-sectional area A_(eff) was 100.0 μm², thedispersion slope was 0.060 ps/nm²/km, the transmission loss when bent ata diameter of 60 mm was 0.210 dB/km (0.215 dB/km or less), and thepolarization mode dispersion was 0.10 ps km^(−½).

Since the optical fiber according to the second embodiment also has alarge positive dispersion in the wavelength band of 1.55 μm, it needsonly a short length for compensating for the negative dispersioninherent in the NZ-DSF in the wavelength band of 1.55 μm, thereby beingsuitable for reducing the dimensions of a dispersion-compensating moduleto which this optical fiber is applied. Also, since this optical fiberhas a large effective cross-sectional area A_(eff) at the wavelength of1.55 μm, nonlinear optical phenomena are effectively restrained fromoccurring. Further, since this optical fiber has a low transmission loss(at the wavelength of 1.55 μm) when bent at a diameter of 60 mm, and itspolarization mode dispersion at the wavelength of 1.55 μm is small, itis suitable for forming a module.

Without being restricted to the above-mentioned embodiments, the presentinvention can be modified in various manners. For example, though sixspecific applied examples are represented as the optical fiber accordingto the first embodiment, and one specific applied example is representedas the optical fiber according to the second embodiment; without beingrestricted thereto, various designs are possible within theabove-mentioned appropriate ranges.

As explained in the foregoing, since the optical fiber according to thepresent invention has a large dispersion in the wavelength band of 1.55μm, it needs only a short length for compensating for the negativedispersion inherent in the NZ-DSF in the wavelength band of 1.55 μm.Consequently, it becomes easy to reduce the dimensions of adispersion-compensating module to which the optical fiber according tothe present invention is applied. Also, since the optical fiberaccording to the present invention has a large effective cross-sectionalarea A_(eff) at the wavelength of 1.55 μm, nonlinear optical phenomenaare effectively restrained from occurring. Further, since this opticalfiber has a transmission loss of 0.215 dB/km or less at the wavelengthof 1.55 μm when wound like a coil at a diameter of 60 mm (furtherpreferably, the original transmission loss of the optical fiberexcluding the increase in loss caused by bending is 0.180 dB/km orless), and its polarization mode dispersion at the wavelength of 1.55 μmis 0.25 ps·km^(−½) or less, it is suitable for forming a module.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

What is claimed is:
 1. An optical fiber comprising: a core extendingalong a predetermined axis; an inner cladding which is a region providedat the outer periphery of said core and has a refractive index lowerthan that of said core; and an outer cladding which is a region providedat the outer periphery of said inner cladding and has a refractive indexlower than that of said core but higher than that of said innercladding; wherein the relative refractive index difference of said corewith respect to said outer cladding is at least 0.30% but not greaterthan 0.50%, the relative refractive index difference of said innercladding with respect to said outer cladding is at least −0.50% but notgreater than −0.02%, the dispersion at a wavelength of 1.55 μm isgreater than 18 ps/nm/km, and the effective cross-selectional areaA_(eff) at the wavelength of 1.55 μm is at least 70 μm².
 2. An opticalfiber according to claim 1, wherein said optical fiber satisfies thefollowing relationships: 2.0≦2b/2a≦6.0 8.3≦2a≦13.0 where 2 a (unit: μm)is the outside diameter of said core region, and 2 b (unit: μm) is theoutside diameter of said inner cladding.
 3. An optical fiber accordingto claim 1, wherein said optical fiber has a dispersion greater than 20ps/nm/km at the wavelength of 1.55 μm.
 4. An optical fiber according toclaim 3, wherein said optical fiber satisfies the followingrelationships: 2.0≦2b/2a≦6.0 9.1≦2a≦13.0 where 2 a (unit: μm) is theoutside diameter of said core region, and 2 b (unit: μm) is the outsidediameter of said inner cladding.
 5. An optical fiber according to claim1, wherein said optical fiber has a transmission loss which becomes0.215 dB/km or less at the wavelength of 1.55 μm when wound like a coilat a diameter of 60 mm, and a polarization mode dispersion of 0.25ps·km^(−½) or less at the wavelength of 1.55 μm.
 6. An optical fiberaccording to claim 1, wherein said optical fiber has an effectivecross-sectional area of at least 90 μm².
 7. An optical fiber accordingto claim 1, wherein said optical fiber has a cutoff wavelength of 1.4 μmor longer at a fiber length of 2 m.
 8. An optical fiber according toclaim 1, wherein said optical fiber has a transmission loss of 0.180dB/km or less at the wavelength of 1.55 μm.
 9. An optical fiberaccording to claim 1, further comprising a carbon coating provided atthe outer periphery of said outer cladding.
 10. An optical fibercomprising: a core region extending along a predetermined axis andhaving an outside diameter of at least 9.5 μm but not greater than 13.0μm; and a cladding region which is a region provided at the outerperiphery of said core and has a refractive index lower than that ofsaid core region; wherein said optical fiber has a relative refractiveindex difference of said core region with respect to said claddingregion of at least 0.3% but not greater than 0.5%, a dispersion greaterthan 20 ps/nm/km at a wavelength of 1.55 μm, and an effectivecross-sectional area of 70 μm² at the wavelength of 1.55 μm.
 11. Anoptical fiber according to claim 10, wherein said optical fiber has atransmission loss which becomes 0.215 dB/km or less at the wavelength of1.55 μm when wound like a coil at a diameter of 60 mm, and apolarization mode dispersion of 0.25 ps·km^(−½) or less at thewavelength of 1.55 μm.
 12. An optical fiber according to claim 10,wherein said optical fiber has an effective cross-sectional area of atleast 90 μm².
 13. An optical fiber according to claim 11, wherein saidoptical fiber has a cutoff wavelength of 1.4 μm or longer at a fiberlength of 2 m.
 14. An optical fiber according to claim 11, wherein saidoptical fiber has a transmission loss of 0.180 dB/km or less at thewavelength of 1.55 μm.
 15. An optical fiber according to claim 11,further comprising a carbon coating provided at the outer periphery. ofsaid cladding region.